Impedance transformation network



Aug. 24, 1954 N, E- LINDENBLAD IMPEDANCE TRANSFORMATION NETWORK Filed March 18, 1952 ATTORNEY Patented Aug. 24, 1954 IMPEDANCE TRANSFORMATION NETWORK Nils E. Lindenblad, Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application March 18, 1952, Serial No. 277,332

The terminal 15 years of the term of the patent to be granted has been disclaimed 9 Claims.

The present invention relates to impedance transformation or matching networks for signal transmission circuits, and more particularly to transformer coupling means for matching two portions of a signal transmission circuit which operate at two different impedance levels.

Transformers of the type capable of handling high power requirements at high frequencies, and which are used as impedance matching networks, generally employ single layer windings, a factor which necessitates resorting to autotransformation in order that conventional ar-' rangements may be employed for achieving nonresonant transformation with a minimum of magnetic leakage. The minimum leakage in such systems, however, is objectionably large.

Accordingly, it is a primary object of this invention to provide an improved impedance matching transformer network suitable for use at high frequencies and having substantially no magnetic leakage.

Generally, in auto-transformation arrangements of the type above mentioned, a portion of the magnetic field of the coil system crosses a coil portion that represents the difference between the primary and secondary windings. As is well known, this represents coupling leakage which manifests itself as a series inductance effect and seriously impairs transformation efliciency and prevents the use of such arrangements over wide frequency ranges.

Accordingly, it is another object of this invention to provide improved auto-transformer impedance matching means substantially devoid of efficiency-limiting series inductance effects and capable of use over wide frequency ranges.

A typical use for which the present invention is adapted is found in radio transmitter stations and the like, where impedance matching between a transmitter output circuit and an antenna input circuit is desired. Where high power must be handled at radio frequencies, such impedance transformation can be done most effectively and economically by air-core transformer means. This is particularly true where transmitterantenna patch cords are employed which have impedance values that must be matched with those of antenna distribution lines. However, the difliculty of designing radio frequency transformers with sufficiently low magnetic leakage has prevented extended use of transmitterantenna patch cord arrangements for carrying high power.

Therefore, it is another object of this invention to provide an impedance matching transformer network with high power carrying ability and with substantially no magnetic leakage whereby it is adapted for connecting antenna and other output signal lines to transmitters.

It is also a further object of this invention to provide an improved auto-transformer impedance matching network which permits the use of transmitter-antenna patch cord arrangements in transmitter stations for carrying high power at radio frequencies.

It is still a further object of this invention to provide an improved auto-transformer type impedance-matching device for transmitter-antenna patch cord systems 'which is inexpensive and occupies a minimum of space.

In accordance with one form of this invention, an improved auto-transformer impedance matching device comprises a pair of coils for coupling respective sides of transmission lines having differing impedance characteristics. Each coil comprises two opposed windings symmetrically arranged on a coil form and connected at their inner ends to provide a center-tap or mid-point for the coil. The outer ends of the windings are connected to ground. The mid-point represents the higher voltage terminal, and connections to a pair of points on the respective windings symmetrical with respect to the mid-point are joined to form the lower voltage terminal. Differential portions of the windings are thus located at the center of the coil system, where substantially no magnetic leakage exists.

The invention will further be understood from the following description with reference to the accompanying drawings, and its scope is pointed out in the appended claims. In the drawings,

Fig. 1 is a block diagram of an impedance matching network used in a transmitter-antemia system in accordance with this invention;

Figure 2 is a schematic diagram of the impedance matching network of Figure 1, showing details of connections between component parts, and

Figure 3 is a simplified schematic diagram of the impedance matching network shown in Figure 2.

Referring to the drawings, in which like reference characters indicate like elements throughout, and referring more particularly to Figure 1, a transmitter I I has a pair of output lines l2, l3, such as a two-wire patch cord, which have a characteristic impedance Z1. An antenna system I4 is provided with a pair of input or distribution lines I5, l6 having an input impedance Z2. An impedance matching network 20 has a pair of input terminals 2 I, 22 to which the respective patch cord lines I2, l3 are connected, and a pair of output terminals 23, 24 to which the respective distribution lines l5, 16 are connected.

The matching network 20 in accordance with this invention permits the use of transmitterantenna patch cords for carrying high power at high frequencies. To this end, and particular attention is here directed to Figure 2, there is provided a pair of symmetrical twin-winding coils 2e, 27 connected between respective input and output terminals 2|, 23 and 22, 24. The coils 26, 21 are identically arranged; consequently the arrangement of the various portions of one coil 26 will be described, and in the drawing, primes of the reference characters used to designate various portions and connections of this coil 26 are employed to designate like or corresponding portions and connections of the other coil 21.

The coil 26 comprises a pair of windings 28, 28 mounted on a coil form 38. The windings 28, 28 are symmetrically arranged on the coil form 30, one winding 28 being a right-hand winding and the other winding 29 being a left-hand winding. The inner ends of the two windings are joined to form a midor center-tap 3| for the coil 23, and their outer ends are joined as indicated to a point of reference, or ground, potential. A pair of taps 32, 33 symmetrical with respect to the center-tap 3i are connected to form a junction 35. The junction 35 is connected to the input terminal 2!, and the center-tap 3| is connected to the output terminal 23.

In a similar manner, the junction 35 of the coil 21 is connected to the input terminal 23, and the center-tap 3! is connected to the output terminal 24.

In single layer winding type auto-transformers employed at high frequencies, the differential portion of the winding, i. e. that portion of the winding which represents the difference between the primary and secondary, is crossed by portions of the magnetic field. Where there is a turnsratio of unity, the portions of the magnetic field crossing the difierential portion of the winding shows up as a shunt inductance. However, where the turns-ratio differs from unity, coupling leakage occurs which shows up as series inductance. Such series inductance has a relatively more serious effect upon the efficiency of the transformer because its effect cannot be easily minimized, whereas the effect of the shunt inductance can be easil minimized by making it sufiiciently large.

In the present invention, however, an attention is here directed to Figure 3, in which the network of Figure 2 is redrawn in simpler form, substantially no change in the distribution of the magnetic field occurs. This can be readily understood upon consideration of the inductance effects of a coil arrangement such as that described herein. Because the opposed halves of each coil are connected in parallel, undesired voltages induced in each winding are equal and opposite, and accordingly are cancelled out at the centertap. Because of the ground and winding connections above described, the differential portion of each coil is effectively located at the center of the coil arrangement, instead of at the ends. In the above described arrangement, the diiferential portion of each coil comprises the portions of the windings thereof between the symmetrically spaced taps 32, 33 and 32', 33. Consequently, substantially all of the magnetic field passes through the differential winding portions, and the series inductance effects heretofore mentioned are avoided.

The impedance on the input side of the matching network 20 is that presented by the parallel combination of the serially connected portions of the windings 28, 29 and the serially connected portions of the windings 29, 23' between the respective taps 32, 33', and 33, 32' and the outer ends of the windings 28, 29 and 29, 28. The output impedance of the network 28 is that presented by the parallel combination of the serially connected windings 28, 29 and the serially connected windings 2 9, 2 8'.

By suitable choice of impedance values for the windings, a desired impedance matching network is obtained which functions in a highly efficient manner.

Because of the substantial lack of the series inductance effects heretofore mentioned, an aircore auto-transformer employed in accordance with the present invention is suitable for efficient impedance transformation at transmission line junctions and permits the use of transmitterantenna patch cord arrangements where high power carrying requirements must be met. An impedance transformation device of the type herein described has been constructed and tested for powers up to 40 kilowatts, for which powers a transmitter patch cord impedance of 240 ohms was transformed to the 550 ohm impedance value of a pair of antenna distribution lines. The device was found to have a wide frequency range comparing favorably with so-called tapered lines. Furthermore, the auto-transformer impedance matching device of the present invention occupies relatively little space, which is a factor of considerable importance at transmitter stations where numerous transmitters are employed.

It will be noted in the arrangement described that the input and output circuits of the network 20 are balanced with respect to ground. However if one of the distribution lines 16 is grounded, as shown in solid lines in Figure l and indicated in dotted lines in Figures 2 and 3, the input circuit of the network will still be balanced with respect to ground.

Obviously, the impedance matching network 20 as described herein can be reversed, with the input terminals 2!, 2'2 being connected to the center-taps 35, El, and the output terminals 23, 24 being connected to the junctions 35, 35, to provide an impedance matching network in accordance with the invention.

From the foregoing description, it is clear that there has been provided an impedance matching network of the air-core auto-transformation type provided with a winding arrangement for preventing magnetic coupling leakage eifects, whereby efiicient impedance transformation in systems handling high power at radio frequencies can be accomplished.

What is claimed is:

1. An inductive impedance matching network for coupling two portions of a two-wire transmission line circuit which operate at different impedance levels, comprising an auto-transformer element for each line of said circuit having two oppositely wound, substantially coaxial windings connected at their adjacent ends to provide a center tap and having the other ends connected together, spaced taps on said windings between said other ends and symmetrically located with respect to said center tap, means connecting said symmetrically spaced taps to a common junction, and connecting means for the junctions and center taps for connecting said junctions to one portion of said circuit and connecting said center taps to the other portion of said circuit.

2. An impedance matching network as defined in claim 1, in which the outer ends of the windings are connected to a point of reference potential, whereby said junctions are balanced with respect to said point.

3. An impedance matching network for matching a first portion of a high power transmission line circuit operating at a first impedance level to a second portion of said transmission line circuit operating at a second impedance level, said network comprising a pair of coils having their end terminals connected to a point of reference potential, a center-tap for each coil, means connecting said center-taps to said second portion of said transmission line circuit, each coil between the end terminals and center-tap comprising oppositely wound, substantially coaxial windings, means providing a circuit junction for each coil, a pair of taps for each coil equally spaced from the center tap thereof and being connected to the circuit junction therefor, and

means connecting the junctions to said first portion of said transmission line circuit.

4. An impedance matching network as defined in claim 3, in which the pair of taps of each coil are symmetrically located with respect to the associated center-tap.

5. An impedance matching network having four terminals adapted to be connected to respective portions of a transmission line circuit which operate at different impedance levels, comprising an auto-transformer type arrangement of two coils each having two substantially coaxial windings connected in opposed inductive relation to a center-tap, the center-taps being connected to two of said terminals, a pair of taps on the respective windings of each coil equally spaced from the associated center-tap, means connecting the equally spaced taps of each coil together to provide a junction, means connecting the junctions to two others of said terminals, and means connecting the ends of each coil to a point of reference potential, whereby said junctions and said center-taps respectively are balanced with respect to said point.

6. The combination with a pair of transmitter output lines operating at a first predetermined impedance level and a pair of antenna distribution lines operating at a second predetermined impedance level, of a coupling network for matching the impedance of the output lines to the impedance of the distribution lines, comprising an air core transformer having a pair of coils each provided with a center-tap and having all of their ends connected to a common point of reference potential, the portions of each coil connected to the center tap being oppositely wound, a pair of terminals located between the ends of each coil and equally spaced from the center tap, a junction for each coil, means connecting the junction of each coil to the equally spaced taps thereon, a pair of terminals connected to the junctions, a pair of terminals connected to the center-taps, one of said pairs of terminals being connected to said output lines, and the other of said pairs of terminals being connected to said distribution lines.

7. The combination as defined in claim 6, in which the terminals for the junctions are connected to the transmitter output lines, and in which the terminals connected to the centertaps are connected to the antenna distribution lines.

8. The combination as defined in claim 6, in which the junction connected terminals are connected to the antenna distribution lines, and in which the center-tap connected terminals are connected to the transmitter output lines.

9. The combination as defined in claim 6, in which the winding portions of each coil are substantially coaxial and relatively tightly inductively coupled.

References Cited in the file of this patent UNITED STATES PATENTS Number 

